Abstract

A colloidal particle placed in an inhomogeneous solution of smaller non-adsorbing polymers will
move towards regions of lower polymer concentration, in order to reduce the free energy of the
interface between the surface of the particle and the solution. This phenomenon is known as diffusiophoresis.
Treating the polymer as penetrable hard spheres, as in the Asakura-Oosawa model, a
simple analytic expression for the diffusiophoretic drift velocity can be obtained. In the context of
drying films we show that diffusiophoresis by this mechanism can lead to stratification under easily
accessible experimental conditions. By stratification we mean spontaneous formation of a layer of
polymer on top of a layer of the colloid. Transposed to the case of binary colloidal mixtures, this
offers an explanation for the stratification observed recently in these systems [A. Fortini et al., Phys.
Rev. Lett. 116, 118301 (2016)]. Our results emphasise the importance of treating solvent dynamics
explicitly in these problems, and caution against the neglect of hydrodynamic interactions or the
use of implicit solvent models in which the absence of solvent back
ow results in an unbalanced
osmotic force which gives rise to large but unphysical effects.